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Apple has updated its Apple Watch support documentation, confirming that the device may have issues when worn by users who have wrist tattoos. The changes were added following a series of reports from new Apple Watch owners who found that their tattoos seemed to interfere with the smartwatch’s ability to track their pulse or cause other problems. Apple now says that permanent and temporary changes to your skin, including the ink used in tattoos, can impact the heart rate sensor’s performance.

In addition, the document clarifies, the ink, pattern and saturation of the tattoo can block the light from the sensor, making it difficult for Apple Watch wearers to get reliable readings. That is to say, those with darker tattoos that cover more of the skin’s surface may have more issues than those with lighter tattoos that are smaller in size.

The paragraph on tattoos was added to a page detailing how the Apple Watch heart rate sensor works, in a section that explains what sorts of factors could affect the sensor’s performance and a wearer’s ability to get a good reading. The Internet Archive, which keeps historical copies of websites, shows that an earlier version of this same page didn’t include the note about tattoos, ahead of the Apple Watch’s launch in April.

That implies that Apple learned of the issues from user feedback, as Watch owners began to call in to report problems with their device. Some users even posted videos to YouTube demonstrating the problem first-hand, which were picked up by the media.

It’s not all that surprising that a wrist tattoo could impact the effectiveness of the Apple Watch’s light sensor. The sensor allows an Apple Watch owner to wear the device looser on the wrist – “snug but comfortable,” says Apple – and still get a good reading. Explains Apple:

Apple Watch uses green LED lights paired with light‑sensitive photodiodes to detect the amount of blood flowing through your wrist at any given moment. When your heart beats, the blood flow in your wrist — and the green light absorption — is greater. Between beats, it’s less. By flashing its LED lights hundreds of times per second, Apple Watch can calculate the number of times the heart beats each minute — your heart rate.

This type of technology has been known to cause problems in the past. For example, a reddit user several months ago noted that they had a problem getting a good reading using the Fitbit HR heart monitor. (A CNet report from 2014 also found that some of the then-current heart rate monitors on the market could also be thrown off by skin pigmentation.)

Apple Watch’s sensor is actually more advanced than the company has claimed, according to the teardown from iFixit posted in late April. The site said that Apple’s heart rate monitor is “actually a plethysmograph—it looks and acts like a pulse oximeter, but Apple isn’t claiming it can measure your blood oxygen level,” iFixit’s analysis noted. It suggested also that Apple wasn’t advertising the functionality due to FDA regulations.

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Despite having this better sensor, it doesn’t solve the problems associated with inks on the skin blocking readings.

Apple recommends a workaround for those who experience these sorts of issues, saying that you can connect your Apple Watch wirelessly to external heart rate monitors, like Bluetooth chest straps. Of course, that’s not quite as elegant a solution as simply wearing a watch, but at least it will give more serious athletes and other quantified self enthusiasts an alternative means of gathering this data.